1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a toothed power transmission belt having a reinforcing fabric layer in the teeth. The invention is further directed to a method of forming the above toothed power transmission belt.
2. Background Art
It is known to make toothed power transmission belts from rubber, liquified polyurethane, etc. Toothed power transmission belts made of polyurethane have excellent resistance to degradation from conditions encountered in an exterior environment. They also have good resistance to wear. As a result, this type of belt has been used as a power transmission device on a bicycle for outdoor use. This type of belt has alternating teeth and grooves along the length thereof, each with a fixed pitch. Load carrying cords made of glass, aramid fibers, or the like, are buried in a backing layer at the pitch line.
This type of belt has no structure that reinforces the teeth at the base thereof near the grooves. Abrasion at the base of the teeth may ultimately lead to damage to the load carrying cord. More specifically, foreign matter such as dust, sand, and the like, may become trapped between the projections on a cooperating pulley and the belt in the groove area with the belt system in operation. This trapped matter tends to abrade the base of the teeth in the groove area and may ultimately cause exposure of the load carrying cord. Once this occurs, the cord is contacted by moisture in normal use. This moisture may diminish the strength of the load carrying cord and ultimately cause premature belt failure.
To avoid this abrasion problem, it is known to place roughly woven fabric at the base of the teeth in a polyurethane synchronous belt. This is taught, for example, in Examined Japanese Utility Model Publication No. 64 12/1971.
Unexamined Japanese Utility Model Publication No. 66648/1988 teaches a polyurethane toothed belt with a non-woven fabric, impregnated with elastomeric material, applied along the base of the teeth.
In Japanese Patent Publication No. 9335/1991, a high load, polyurethane, toothed belt is disclosed with the teeth and the base thereof covered with a canvas layer. A load carrying cord therein is made of aramid cord type II having a tensile elasticity rate of approximately 1.3.times.10.sup.6 kg/cm.sup.2 in greige filament.
Examined Japanese Patent Publication No. 62657/1993 discloses another polyurethane toothed belt wherein a slightly compressed, non-woven fabric impregnated with elastomeric material is applied at the base of the teeth in the grooves between adjacent teeth. The non-woven fabric extends into and is dispersed within the teeth to effect reinforcement thereof. The entire tooth is reinforced, even at the inside thereof where compressive forces are the lowest.
The prior art polyurethane toothed belts having a canvas cover layer on both the belt teeth and the base of the teeth in the grooves permit high load transmission since the canvas protects the teeth and the base thereof such that little damage occurs to the load carrying cord after use. However, in manufacturing this belt, the canvas cannot be properly adhered to the tooth surfaces without preapplying the canvas conformingly in the grooves in a metallic mold.
Unexamined Japanese Patent Publication No. 246532/1988 discloses a method for applying a bulky, non-woven fabric over the grooves of an internal mold by wrapping a load carrying cord therearound prior to injection of a liquified polyurethane into the mold.